In the past, coating compositions were commonly prepared by dissolving or dispersing film-forming organic polymers in volatile organic compounds (VOC). Environmental and health concerns associated with applications involving large-scale vapor emissions have led to research in the development of coating compositions wherein the emission of volatile organic compounds is minimized.
Among the various methods of reducing the vapor emissions of coating compositions, the use of reactive diluents to replace all or part of the volatile organic solvent component of a coating composition is of particular interest. As used herein, the term "reactive diluent" refers to nonvolatile or nearly non-volatile organic solvents or dispersants having as an integral part of their molecular structure, functional groups which are reactive with a film-forming polymer and/or cross-linking agent.
In addition to providing a low level of volatile emissions, a coating composition should have a sufficiently low viscosity to permit easy handling and application. Other desirable properties in a coating composition are sufficient stability to ensure a commercially acceptable shelf life and the ability to provide a cured coating having suitable properties such as toughness, adhesion, scratch resistance, weatherability, and resistance to attack by solvents, acids, bases and other chemicals. Coating compositions wherein all or a portion of the volatile organic solvent component thereof is replaced by a reactive diluent are illustrated by the following patents:
U.S. Pat. No. 4,417,022, to Chang et al. discloses coating compositions, the vehicle portion of which consists essentially of from about 60 percent to about 97 percent of a curable film forming component and from about 3 percent to about 40 percent of an organic reactive diluent capable of chemically combining with the curable film-forming component. Disclosed as reactive diluents are ethers having less than five ether groups, amides, esters, urethanes, ureas, sulfur-containing compounds, and mixtures thereof which have one primary or secondary hydroxyl group. The reactive diluents disclosed by Chang et al, are further characterized as having a retained solids value of greater than about 80 percent, a hydroxyl equivalent of from about 180 to about 800, and a liquid viscosity of less than about 10 poise at 60.degree. C. Preferred reactive diluents disclosed by Chang et al are ester-containing reactive diluents, with ester-containing reactive diluents having allyl side chains being most preferred. Coating compositions having ester-containing reactive diluents, however, tend to produce cured coatings which lack desirable adhesion, hardness and/or weatherability
As a further example, U.S. Pat. No. 4,520,167 to Blank et al., discloses a coating composition comprising (a) a hydroxyalkyl carbamate of the formula: ##STR1## wherein n is 0 or 1, R is a C.sub.1 to C.sub.20 organic moiety which may contain one or more constituents selected from the class consisting of hetero-atoms and hydroxyl groups, and each of R.sub.1, R.sub.2 and R.sub.3 is independently H or CH.sub.3 ; (b) an aminoplast cross-linker; and (c) a polymer containing active sites which at elevated temperatures are reactive with the amide-aldehyde crosslinker (b).
Cured coatings formed from the coating compositions containing the reactive diluents exemplified by Blank et al. are, within a range of environmentally acceptable formulations, widely variable as regards the surface properties possessed by same.
Among the compounds which have been offered for use as reactive diluents in coating compositions are dicyclopentenyl-oxyethyl ethyl methacrylates, modified caprolactones, and unsaturated melamines. In general, these compounds are poor solvents, have relatively high viscosities and/or produce cured coatings having undesirable chemical and/or physical properties.
A cured coating's toughness, adhesion, impact resistance, abrasion resistance, scratch resistance, weatherability and resistance to chemical attack depend to a large extent upon the film-forming polymer and reactive diluent and cross-linker components of the composition used to produce same. As disclosed in U.S. patent application Ser. No. 759,172 filed on Jul. 26, 1985, now abandoned and U.S. patent application Ser. No. 807,738 filed Dec. 11, 1985, now abandoned both in the name of K. L. Hoy, et al., coating compositions containing carbamate and urea derivatives as reactive diluents are found to provide cured coatings having desirable toughness and adhesion.
As indicated above, industrial use of coatings is being increasingly limited in the amount of volatile organic compounds it can release into the atmosphere by government regulations intended to protect the environment and those who apply the coatings. This increasing pressure for change has created an opportunity for new technologies with potential for lower VOC. As mentioned above, one of these is the use of reactive diluents or cosolvents in place of all or part of the volatile organic solvents of the prior art. Many early attempts to provide reactive cosolvents or diluents have failed because the resulting coating compositions suffered from degraded application or end-use properties. In fact, there have been very few if any commercially significant implementations of reactive cosolvents to date.
In the present invention, the earlier deficiencies have been borne in mind and new classes of compounds evaluated which would confer useful application and ultimate properties on coating compositions and the coatings derived therefrom. As previously indicated, these properties include proper wetting, rheological and shelf stability of the coating composition, and adhesion, flexibility, hardness and resistance to weathering and solvents of the cured product.
In accordance with the present invention, the synthesis of a basically different class of reactive diluents was undertaken. These diluents are called "AmBifunctional" or "AB"- type materials because they combine in one molecule balanced amounts of the two different types of reactivity ("A" and "B") usually segregated into different molecules in the traditional film-forming ingredients of coating compositions. The prior art materials evaluated to date in the search for new reactive diluents can be classed as type "A" or "AA" or multi-"A" materials in which the "A"'s stand for two or more reactive nucleophilic or acidic hydrogen groups, such as, OH or NH groups. These "A" groups react with type "B" groups or electrophilic groups during the cure of the coating. Example of "B"-type groups are the alkoyxmethylene and hydroxymethylene amino groups of aminoplast crosslinkers.
The compounds depicted by structural formula below, illustrate the types of compounds which have "A" and "B" reactive groups. The particular reactive groups has been identified by A or B in a circle. ##STR2##
Thus, as indicated above, most of the materials studied to date in the search for new reactive diluents can be classed as type "A" or "AA" or multi-"A" materials in which the A stands for two or more reactive nucleophilic groups. These "A" groups react with "B" or electrophilic groups during the acid catalyzed cure of the coatings.
A discussion of aminoplast coating cure reactions of the prior art, will aid in understanding the "AB" reactive diluent concept and its value in coatings. Under acid catalysis (e.g. p-toluenesulfonic acid; PTSA) the aminoplast (e.g. Cymel-303) is protonated and dissociated or partially dissociates into alcohol (methanol in the case of Cymel-303) and an iminium ion. The iminium ion can then react with other alcohols while the more volatile methanol is removed from the matrix by evaporation. If the average effective functionality of the coating ingredients exceeds two, there will be crosslinking resulting in an insoluble composition. The overall chemical reaction is set forth below wherein the iminium ion is similar to an isocyanate in its reactivity with OH and NH groups. ##STR3## Subsequent reaction of the iminium ion with a film-forming polymer can be shown as follows: ##STR4## These "AA" molecules have certain deficiencies from the viewpoint of the coatings technologist. These deficiencies stem from the fact that the reactive diluents have a low equivalent weight and, thus a large requirement for crosslinker relative to that of the film-forming polymer. This low equivalent weight is a consequence of having at least two reactive groups in a molecule small enough to have reasonably low viscosity and high solvency. Thus, when the coatings technologist wishes to adjust the amount of reactive solvent in a coating composition to achieve final application viscosity, he may be required to change the amount of the crosslinker (e.g., Cymel aminoplast or a polyisocyanate), to compensate for the added reactive groups of the solvent.
The adjustment to the amount of crosslinker will also change the viscosity of the composition. Thus optimization of the formulation is complicated by use of this stoichiometrically unbalanced, "AA" type of reactive diluent and adjustments to viscosity can become an iterative process. Another undesirable consequence of having to add more crosslinker to balance the reactive diluent is that it brings more potential for methanol emissions into the coating formulation. This is because the functionality of a melamine crosslinker is typically less than or equal to four but it can bear more than the four groups that actually participate in forming coating crosslinks. Thus, it would be much more desirable to be able to vary the amount of the reactive diluent widely without having to vary the amount of crosslinker. It was to fill this need for a reactive diluent that would not upset the stoichiometry of a coating composition that the concept of the "AmBifunctional" reactive diluent was conceived. Rather than having two or more reactive OH or NH functional groups which can react with the crosslinking agent, such as an aminoplast, an epoxide or a blocked isocyanate, the new "AmBifunctional" compounds of the present invention combine equal amounts of the chemical reactivity of a crosslinker (or electrophile) and an active hydrogen compound (or nucleophile) in the same molecule. Thus their use does not require that the ratio of crosslinker to film-forming polymer be adjusted. Initially, materials having one each of the OH or NH (type A, nucleophilic) groups and methylolamido or methoxymethyleneamide groups (type B, electrophilic) have been prepared. These AB molecules are valuable diluents because they confer on the cured coating compositions the advantages of the "AA" molecules which have previously been employed without radically changing the coating composition's stoichiometry as the "AA" materials do.
Accordingly, one or more of the following objects will be achieved by the practice of the present invention. It is an object of this invention to provide AmBifunctional reactive diluents which are useful in the preparation of high solids coatings. Another object of this invention is to provide AmBifunctional reactive diluents which do not adversely affect a coating composition's stoichiometric ratio of nucleophilic and electrophilic groups. A further object of this invention is to provide certain carbamates, ureas and amides which are suitable as stoichiometrically balanced reactive diluents Another object of this invention is to provide a process for the preparation of coating compositions. A still further object of the present invention is to provide cured coating compositions prepared from formulations containing the reactive diluents. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.